1. Field of the Invention
The present invention relates generally to the removal of sulfur dioxide from effluent gas streams. More particularly, the invention relates to the removal of sulfur dioxide (SO.sub.2) from flue gas streams by the use in a scrubber of a recirculating aqueous stream containing magnesium sulfite as a strong sulfur dioxide absorbent. The magnesium sulfite is derived from magnesium hydroxide, which in turn is derived from the reaction of soluble magnesium in seawater with calcium hydroxide.
Sulfur is found in a wide variety of fossil fuels in greater and lesser amounts. When burning the fuels, oxidation of the sulfur results in formation of sulfur dioxide, which is a serious pollutant. The sulfur dioxide forms sulfurous acid when mixed with water and, upon oxidation, will form sulfuric acid. Together, these acids are believed to be a primary cause of "acid rains" which have been responsible for injuries to the environment. It would therefore be desirable to provide methods and apparatus for preventing or inhibiting the release of sulfur dioxide into the atmosphere when burning sulfur-containing fossil fuels.
Three primary approaches have been proposed for the control of the sulfur dioxide emissions. The first approach relies on the removal of sulfur from the fuel prior to combustion. The second approach relies on the removal of sulfur dioxide during combustion, typically by injection of alkali into the combustion chamber of a furnace. The third approach relies on the removal of sulfur dioxide from the combustion gases after burning of the fuel. The present invention relates to the latter approach, and in particular relates to the reaction of magnesium from seawater with hydrated lime to form magnesium hydroxide which is used to chemically absorb sulfur dioxide from the combustion effluent gases.
The use of seawater for flue gas desulfurization has been previously proposed. In most cases, the natural alkalinity of seawater is relied on to desulfurize the combustion effluent in a single-pass system. Although effective, the use of seawater alone requires a very large flow to remove a high percentage of the sulfur dioxide. For power plants which utilize spent cooling water from the turbine condensers as the seawater source, it would be necessary to employ from 20% to 100% of the total cooling water flow in order to remove a high percentage of sulfur dioxide. The equipment and operating costs for handling such large volumes of water can be prohibitively expensive. Moreover, the seawater used to absorb sulfur dioxide will also absorb large amounts of heat, which results in a temperature rise which can be detrimental to the marine environment. Neutralization and oxidation of the scrubber effluent will require additional treatment capability, further increasing the capital and operating costs of the process.
It would thus be desirable to provide methods and systems which employ seawater to efficiently scrub sulfur dioxide-containing gases. In particular, such methods and systems should be able to function with relatively low volumes of seawater; e.g., less than 2%, preferably less than 1%, of the spent cooling water flow from the turbine condensers of a power plant in order to treat the entire effluent gas flow. It would further be desirable if the aqueous effluent from such a treatment process could be returned to the marine environment without further treatment, and that the process would not require the separation of solid wastes at any point. Finally, it would be highly desirable if the process resulted in only a very limited temperature rise in the seawater being returned to the marine environment, preferably less than 1 degree C, more preferably less than 0.5 degree C.
2. Description of the Background Art
U.S. Pat. No. 4,085,194 describes the use of a large flow of seawater (spent cooling water) to remove sulfur dioxide (SO.sub.2) from flue gases. As the natural alkalinity of the seawater is relied on for such removal, it would be necessary to use most or all of the spent cooling water flow from the turbine condensers to effect a high percentage of desulfurization in a power plant burning coal having a sulfur content of from about 1.0% by weight or higher. The SO.sub.2 -containing seawater must then contacted with an oxygen-containing gas for removing carbon dioxide in order to increase the pH and oxidize sulfite ions to sulfate ions prior to return of the seawater to the marine environment.
U.S. Pat. No. 4,337,230 describes the addition of calcium oxides to seawater subsequent to sulfur dioxide absorption. Approximately 20% of the cooling water from the turbine condensers of a power plant would be required for high percentage desulfurization on a once-through basis. Even when recombined with the remaining 80% of the cooling water, a significant temperature increase occurs which could affect the marine biology. Moreover, the capital and operating costs of the system are very high, and sulfur dioxide will be released as a result of oxidation of the very low pH seawater effluent. Finally, full oxidation of the seawater effluent is difficult to achieve, leaving significant chemical oxygen demand which could be harmful to the ocean environment.
Litter, "Flue Gas Washing at Power Stations in the U.K. 1933-1977," July 1976, discussed in U.S. Pat. No. 4,337,230, describes the use of spent cooling water to scrub sulfur dioxide from a power plant flue gas on a once-through basis. Approximately 10% of the total cooling water flow is required, and limestone (calcium carbonate) is added to the cooling water to increase its absorptive capacity. After scrubbing, the absorbed sulfites are oxidized to sulfates by aeration in the presence of a manganese sulfate catalyst, and the aerated water is recombined with the remaining 80% of the cooling water prior to discharge. The method requires that the calcium sulfite and calcium sulfate reaction products, and excess calcium carbonate, be separated by filtration prior to discharge. Moreover, the final effluent has a pH of about 2.3, allowing substantial release of sulfur dioxide from the water. Finally, the effluent will have substantial chemical oxygen demand because of the difficulty of providing complete oxidation at low pH.
Japanese Pat. No. 49-52762 describes the addition of milk of lime to seawater, used as a cooling water stream, to obtain magnesium-free seawater and a magnesium hydroxide slurry. The magnesium hydroxide slurry is used to treat a sulfur dioxide-containing waste gas, resulting in a magnesium sulfite-containing slurry. The magnesium sulfite solution is separated from the magnesium hydroxide slurry, oxidized to magnesium sulfate, and combined with the magnesium-free seawater. The slurry is recycled. Although this approach is theoretically possible, separation of very fine particles of magnesium hydroxide from the slurry is very difficult to achieve in practice.
U.S. Pat. No. 4,246,245 describes the use of Type S hydrated lime as an absorbent for sulfur dioxide in a wet scrubber, spray dryer, or boiler injections system. Type S hydrated lime includes both calcium and magnesium hydroxides, where the magnesium hydroxide is converted to magnesium sulfite in the presence of the sulfur dioxide.
See also U.S. Pat. Nos. 3,622,270; 3,650,692; 3,653,823; 3,904,742; 4,046,856; and 4,623,523; and Japanese Pat. Nos. 49-110570; 55-73326; and 80-25892 which relate to flue gas desulfurization.